Laser encoder device

Abstract

A laser encoder device is disclosed into which an optical fibre assembly (10) is installable. The device comprises a seating arrangement (60) for receiving and holding a component (17) of the assembly (10) within the device. The seating arrangement (60) is adapted to constrain the component (17) kinematically relative to the device such that the assembly (10) can be removed from and reinstalled into the device with a stable and / or repeatable alignment of the component (17) relative to the device.

Description

[0001] Laser Encoder Device

[0002] The present invention relates to a laser encoder device. The present invention relates in particular, but not exclusively, to improvements in the setup and operation of such a laser encoder device.

[0003] Figure 1 of the accompanying drawings illustrates a fibre optic laser encoder system 1 which is made and sold by Renishaw pic. The laser encoder system 1 provides position feedback signals suitable for use in precision position feedback applications such as machine calibration and motion control.

[0004] The main components of the laser encoder system 1 are a laser unit 2, a detector head (or detector unit) 4 and a machine interface 6. The detector head 4 is the core of the optical measuring system and will be described in further detail below with reference to Figure 2 of the accompanying drawings. The laser unit 2 comprises a laser source and signal processing electronics, with an electrical cable 3 for receiving signals from and providing power to the detector head 4 and a fibre optic conduit 5 that delivers laser light directly to the detector head 4 through a fibre optic cable (not visible in Figure 1) within the fibre optic conduit 5. The machine interface 6 forms part of a controller 31 and communicates with the laser unit 2 via an electrical cable 7.

[0005] To complete the configuration, a target optic 8 is provided in the path of a laser beam 9 emitted from the detector head 4, such that the laser beam 9 is reflected off the target optic 8 and returned to the detector head 4. In this example, the target optic 8 is in the form of a retroreflector but with a variant of the detector head 4 the target optic 8 could instead be a plane mirror. The beam 9 is a measurement beam, with the return measurement beam interfering with a reference beam which in this example is internal to the detector head 4, with the distance to the target optic 8 (or rather changes in this distance relative to a chosen datum position) being determinable from the interference signal in a known way.

[0006] Figure 2 is a schematic illustration of the main components of the detector head 4 of Figure 1. The fibre optic cable 11 passes through into the body or housing 24 of the detector head 4 and into a collimator 17. The fibre optic cable 11 is terminated within and held in place via a ferrule 21, with laser light being emitted from the end of the fibre optic cable 11 in a diverging cone. The role of the collimator 17 is to collimate this diverging beam, using a lens 23, before it passes further through the detector head 4. As shown by the arrows, the collimated beam passes first to a beam splitter 14, with some of the light being reflected up to a reference retroreflector 18 (as a reference beam) and the remainder of the beam passing out through the laser aperture 16, via a beam steerer 30, and onwards (as the measurement beam 9) to the retroreflector target optic 8. Also shown in Figure 2 is a circuit board 12, which supports various processing, detection, and control electronics (such as a light detector 19), as well as an optical shutter 26 which can be used to shut off the measurement beam 9.

[0007] The return measurement beam from retroreflector target optic 8 re-enters the detector head 4 via the laser aperture 16, and through the beam splitter 14 where it joins (and interferes with) the measurement beam from the reference retroreflector 18 and is incident on a light detector 19. An analogue quadrature interference signal (or detection signal) from the light detector 19 then passes out from the detector head 4 via the electrical cable 3 where it is received at the laser unit 2 shown in Figure 1. Although there may be some processing performed on the interference signal at the laser unit 2, the main processing is typically performed at the interface 6, having received the interference signal from the laser unit 2 via electrical cable 7.

[0008] By digitising, interpolating and processing the interference signal the interface 6 can determine with high accuracy how far the retroreflector target optic 8 has moved by counting fringes, or rather pulses in the digitised / interpolated version of the signal. The positional data from the interface 6 can then be used by the controller 31 for the intended purpose, such as machine calibration or motion control. It should be noted that the laser unit 2 can be set to output a digital rather than analogue quadrature output signal, in which case the digitising and interpolating would be performed at the laser unit 2 rather than at the interface 6.

[0009] The exterior of the detector head 4 of Figure 1 is shown in more detail in Figure 3 of the accompanying drawings. The fibre optic cable 11 can just be seen within the fibre optic conduit 5. The fibre optic conduit 5 is itself coupled to the body of the detector head 4 via a strain relief 15, which is intended to prevent or at least limit forces on the fibre optic conduit 5 and the enclosed fibre optic cable 11 being transferred to any internal optical components to which the fibre optic cable 11 is connected. A laser aperture 16 is also apparent in Figure 3, through which both the outgoing and returning measurement beams 9 will pass.

[0010] The detector head 4 as shown in Figures 1 to 3 is just one type of detector head made and sold by Renishaw pic. Figure 4 of the accompanying drawings shows another type of detector head 4, which differs from that shown in Figure 3 mainly in that the measurement beam 9 is emitted at a ninety-degree angle to main axis of the detector head 4 (rather than zero-degree angle). The detector head 4 of Figure 4 would typically be used as a pair, with one detector head 4 of the pair measuring along an X machine axis and the other measuring along a Y machine axis, and would typically use a target optic 8 in the form of a plane mirror (though a retroreflector target could also be used). It can be seen that the laser unit 2 of Figure 1 has a spare set of connections available for accommodating a second detector head 4 in this way.

[0011] Figure 5 of the accompanying drawings shows yet another type of detector head 4, which differs more substantially from that of Figures 3 and 4. The detector head 4 of Figure 5 is a differential interferometer detector head 4, with a pair of measurement beams 9M emitted from the laser aperture 16 as well as a pair of reference beams 9R. By using an external reference beam 9M, the differential interferometer detector head 4 is able to measure the relative displacement between two plane mirror targets, one of which (the reference target) would typically be in a fixed position, for example on a fixed column of the machine, and the other of which (the measurement target) would be moving, for example on a moving stage of the machine on which a semiconductor wafer or other workpiece is supported. This helps to ensure accurate positioning between process critical components and to eliminate common mode errors.

[0012] For a detector head 4 of a type as shown in Figures 3 and 4 the electrical cable 3 is fixedly connected to the detector head 4, but for a detector head 4 of a type as shown in Figure 5 an electrical connector 13 is provided for releasably connecting the electrical cable 3 to the detector head 4. For all of these detector heads 4 the electrical cable 3 is releasably connectable at the other end to the laser unit 2. On the other hand, for all of these detector heads 4 the fibre optic conduit 5 (with enclosed fibre optic cable 11) is detachable from the detector head 4 (as will be explained in more detail below) but is fixedly coupled into the laser unit 2. In this respect, the fibre optic cable 11 is continuous from the fibre launch within the laser unit 2 all the way to the collimator 17 and therefore cannot easily be disconnected from the laser unit 2. The detector head 4 typically incorporates a feature that prevents the laser beam 9 being emitted if either the fibre optic cable 11 or electrical cable 3 is disconnected.

[0013] The collimator 17, the strain relief 15, fibre optic conduit 5 (with enclosed fibre optic cable 11) and fibre optic connector 34 (see Figure 1) can be considered to form a single optical fibre assembly 32, with the collimator 17 being considered as an integral part of the optical fibre assembly 32 because it terminates the fibre optic cable 11 in a manner required by the detector head 4 (and the laser encoder system 1 as a whole). The components of the optical fibre assembly 32 (and the internal components of the collimator 17) are assembled and aligned precisely in the manufacturing facility, and supplied to the customer as a unit, and as such are considered to be inseparable in normal use. The way in which the detector head 4 is adapted to receive the optical fibre assembly 32 will be more apparent from Figures 6 and 7 of the accompanying drawings.

[0014] When the laser encoder system 1 is being configured for operational use, the supplied optical fibre assembly 32 is connected to the detector head 4 simply by pushing the collimator 17 through a correspondingly sized opening formed in the housing 24 and into the body of the detector head 4, with the strain relief 15 remaining outside the body and in contact with the housing 24. This connection operation is illustrated in Figure 6 for a differential interferometer detector head 4 of a type shown in Figure 5, but the connection would be entirely equivalent for a detector head 4 of a type shown in Figures 3 and 4. The combination of the strain relief 15 and collimator 17 can be referred to as a fibre barrel 10, with the optical fibre assembly 32 being terminated by the fibre barrel 10.

[0015] Figure 7 shows a view of the differential interferometer detector head 4 of Figure 4 with the upper part (or lid) of the housing 24 removed. This illustrates how the collimator 17, having been pushed into the body of the detector head 4, is held in place tightly by a clamp 20, which is in turn tightened via a locking screw 22. It will also be apparent from Figures 6 and 7 that the collimator 17 is rigidly coupled to the strain relief 15, consistent with the above explanation that the collimator 17 and the strain relief 15 form part of a unitary optical fibre assembly 32. The strain relief 15 and collimator 17 are prevented from being pulled away from the detector head 4 by action of the clamp 20 on the collimator 17, but there could be an additional coupling of the strain relief 15 to the housing 24 (e.g. via a screw thread connection).

[0016] For a proper connection, the fibre barrel 10 (and in particular the collimator 17) must be inserted into the detector head 4 in the correct orientation. To achieve this, and as shown in Figure 6, a line L marked on the fibre barrel 10 is aligned visually with an alignment dot D marked on the detector head 4. The fibre barrel 10 is inserted fully and then rotated slightly until a ball bearing inside the clamp 20 can be felt to engage in a recess 38 in an outer surface of the collimator 17 (specifically in a lid of the collimator 17), with the ball bearing being biased by a spring 39 (see Figure 7) into the indent 38 to form a detent feature. There would also be an end stop in the lid of the detector head 4 which sets the position along the insertion axis, so that the indent 38 is in the correct axial position to receive the ball bearing that is biased into it by the spring 39.

[0017] The present applicant has appreciated that the way in which the collimator 17 is received into the body of the detector head 4 and held in place by the clamp 20 can cause some issues relating to the configuration of the detector head 4 for operational use. For example, it is important to ensure that the collimated beam emitted from the collimator 17 is aligned correctly relative to other components within the detector head 4 that lie along the optical path of the beam. It may be that correct alignment must be achieved not only in respect of pitch and yaw but also in respect of roll.

[0018] Regarding pitch and yaw alignment, although the direction of the measurement beam 9 emitted from the detector head 4 can be altered above using the beam steerer 30, it is also important that the output beam from the collimator 17 itself is aligned correctly at setup, and that this alignment is maintained during operational use. Regarding roll alignment, the light in the output beam may be polarised, it may be that an axis of polarisation of the output beam must be aligned with an optical component (such as a polariser) within the detector head 4.

[0019] The present applicant has appreciated that it can be challenging to achieve the correct alignment of the collimated beam from the collimator 17 when setting up the detector head 4, for example with the alignment being sensitive to the degree of tightening applied to the clamp 20 by the screw 22. The present applicant has appreciated that there is also the potential for the alignment to drift should the clamp 20 become loosened over time. The present applicant has also appreciated that it would be beneficial if the optical fibre assembly 32 could be easily removed and re-inserted into the detector head 4 without having to perform a complicated alignment setup each time. The present applicant has also appreciated that it would be beneficial to have the flexibility to be able to insert the same optical fibre assembly 32 into different detector heads 4 of the same type or different optical fibre assemblies 32 of the same type into the same detector head 4, all without having to perform a complicated alignment each time.

[0020] With the above in mind, the present applicant has appreciated that it would be desirable to make some improvements to the detector head 4 in order to enhance operational performance and stability, as well as the overall ease of use.

[0021] According to a first aspect of the present invention there is provided a device into which an optical assembly is installable, the device comprising a seating arrangement for receiving and holding a component of the assembly within the device, and the seating arrangement being adapted to constrain the component kinematically (or at least pseudo kinematically) relative to the device. The kinematic (or pseudo kinematic) nature of the coupling enables the assembly to be installed into the device with a stable alignment of the component relative to the device and / or enables the assembly to be removed from and reinstalled into the device with a repeatable alignment of the component relative to the device. As explained in more detail below, kinematic constraint can alternatively be referred to as exact constraint.

[0022] In some use cases, the stability provided by the kinematic nature of the constraint would be more important than the repeatability provided by the kinematic nature of the constraint, but in other use cases it would be the opposite, or both would be equally important. But it can be considered that both properties (stability and repeatability) stem from the kinematic nature of the coupling. The stability of the coupling overcomes the above-mentioned issued associated with positional drift over time, while the repeatability of the coupling allows the optical assembly to be removed and re-inserted into the device (or another device embodying the present invention) without having to perform a complicated alignment setup each time.

[0023] The device may be a laser encoder device.

[0024] The optical assembly may be an optical fibre assembly.

[0025] The seating arrangement may be adapted to constrain the component kinematically in at least three degrees of freedom.

[0026] The seating arrangement may be adapted to constrain the component kinematically in at least four degrees of freedom.

[0027] The seating arrangement may be adapted to constrain the component kinematically in exactly four degrees of freedom.

[0028] The seating arrangement may be adapted to constrain the component kinematically in at least two rotational and at least two translational degrees of freedom.

[0029] The seating arrangement may be adapted to constrain the component kinematically in exactly two rotational and two translational degrees of freedom.

[0030] The seating arrangement may be adapted to constrain the component kinematically in all three rotational degrees of freedom.

[0031] The seating arrangement may be adapted to hold the component within (and / or guide the component into) a predetermined range of angles in a third rotational degree of freedom. This is preferably done without impacting the kinematic constraint in the other degrees of freedom.

[0032] Constraint in the third rotational degree of freedom may be provided by a post projecting from a surface of the component which is received into a slot in the seating arrangement.

[0033] The kinematic coupling may be unconstrained in a translational degree of freedom to allow the assembly to be installed into and removed from the device.

[0034] At least some resistance to translation in the unconstrained degree of freedom may be provided by a frictional force from a biasing member which provides a nesting force for the kinematic coupling.

[0035] The frictional force may be arranged to be greater than a gravitational force due to the weight of the component and anything supported by it. This would help to prevent the assembly falling out of the device.

[0036] The biasing member may be arranged to move or lift out of the way of the component as it is installed into the device.

[0037] The unconstrained translational degree of freedom may define a seating axis for the seating arrangement. The alignment of the component relative to the seating axis may be repeatable with each reinstallation of the assembly into the device.

[0038] A translational axis of the unconstrained translational degree of freedom may be aligned with a rotational axis of the third rotational degree of freedom.

[0039] The device may comprise an opening through which the assembly is inserted or pushed to install it into the device. The assembly may be installed into the device through the opening, with the component being received and held within the device by the seating arrangement, without having to open the device (e.g. to remove or lift a lid or a part of the housing or enclosure).

[0040] The assembly may be inserted or pushed into the device in a direction that is substantially aligned with the unconstrained translational degree of freedom.

[0041] The laser encoder device may be a detector head (or detector unit).

[0042] The assembly may comprise a fibre optic cable for providing light to the device in use. The fibre optic cable may be enclosed by a fibre optic conduit.

[0043] The component may terminate the assembly.

[0044] The component may be an optical or metrology component.

[0045] The component may be a collimator.

[0046] The collimator may be coupled to a strain relief component.

[0047] The collimator and strain relief component may form a fibre barrel component.

[0048] According to a second aspect of the present invention there is provided a method of installing an optical fibre assembly into a laser encoder device as claimed in any preceding claim, comprising inserting the assembly into an opening in the device such that the component is coupled kinematically into the device via the seating arrangement, removing the assembly from the device, and inserting the assembly into the device again such that the component is again coupled kinematically into the device via the seating arrangement with a repeatable alignment of the component relative to the device.

[0049] Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1, discussed hereinbefore, illustrates a known fibre optic laser encoder system comprising a laser unit, detector head and machine interface;

[0050] Figure 2, also discussed hereinbefore, is a schematic view of some of the internal components of the detector head of Figure 1;

[0051] Figure 3, also discussed hereinbefore, shows in more detail a detector head of the type shown in Figures 1 and 2;

[0052] Figure 4, also discussed hereinbefore, shows a different type of detector head in which the laser beam is emitted from the body at a different angle;

[0053] Figure 5, also discussed hereinbefore, shows a differential interferometer type of detector head which emits both measurement and reference beams;

[0054] Figure 6, also discussed hereinbefore, shows how the fibre barrel of an optical fibre assembly is connected to a differential interferometer type of detector head;

[0055] Figure 7, also discussed hereinbefore, shows a view of the detector head of Figure 6 with the upper part of the housing removed;

[0056] Figure 8 is a schematic illustration of a detector head having a seating arrangement according to an embodiment of the present invention, with the collimator being seated onto the seating arrangement;

[0057] Figure 9 shows a schematic end view of the seating arrangement and collimator when looking in the direction into the detector head;

[0058] Figure 10 shows a schematic plan view of the seating arrangement and collimator when looking down onto the detector head, before the collimator has been inserted into the detector head;

[0059] Figure 11 shows a schematic plan view of the collimator when seated onto the seating arrangement, showing the contacts made between the collimator and the support members of the seating arrangement;

[0060] Figure 12 shows a seating arrangement having an alternative arrangement of three support members rather than four;

[0061] Figure 13 shows a perspective view of the collimator biased onto the seating arrangement by the arm in a less schematic embodiment of the present invention;

[0062] Figure 14 shows a view from a slightly different angle to that of Figure 13, and also shows the strain relief and part of the housing;

[0063] Figure 15 show a view looking down on the seating arrangement of Figure 13 and more clearly shows the support members of the seating arrangement;

[0064] Figure 16 is an end view of the collimator and seating arrangement of Figure 13 looking in a direction from inside the detector head to out; and

[0065] Figure 17 is a view to help explain the determination of a suitable spring force and lever distance for the biasing member for the embodiment of Figure 13.

[0066] Figure 8 is a schematic side view of a fibre barrel 10 which has been inserted into a laser encoder device (or detector head) embodying the present invention. The fibre barrel 10 comprises a strain relief 15 and a collimator 17, with these parts being generally similar in overall function to those described above so that a detailed further description of them is not required. The fibre barrel 10 of Figure 8 has a two-piece type of construction, with the strain relief 15 being physically decoupled from the collimator 17 by virtue of a gap G between them. The decoupling feature used to achieve the decoupling of the strain relief 15 from the collimator 17 is not described herein but is explored further in our co-pending application filed on the same date as the present application.

[0067] The fibre barrel 10 is inserted into the detector head 4 with a push on the strain relief 15 in the direction of the arrow P, with the strain relief 15 being pushed as far as a housing 24 of the detector head 4, and in this position the collimator 17 is located inside the detector head 4. A seating arrangement 60 is adapted to hold the collimator 17 in place within the detector head 4 after insertion, and by providing a biasing force to the collimator 17 as part of this function it also offers some frictional resistance to the movement of the collimator 17 during insertion.

[0068] The features of the seating arrangement 60 will now be discussed with reference to Figures 8 to 11. Figure 9 is a schematic end view of the seating arrangement 60 and collimator 17 when looking in a direction into the detector head 4, i.e. from left to right in Figure 8, with the collimator 17 being located into the seating arrangement 60. Figure 10 is a schematic plan view of the seating arrangement 60 and collimator 17 when looking down onto the detector head 4, i.e. from top to bottom in Figure 8, but with the collimator 17 not yet located onto the seating arrangement 60. For the sake of simplicity, roll alignment features 66 and 67 from Figures 8 and 9 are not shown in Figure 10.

[0069] As will be most apparent from Figure 10, the seating arrangement 60 comprises four support members 62, 63, 64 and 65, which in this embodiment are each in the form of a sphere or ball (or which have an at least part spherical surface where it contacts the surface of the collimator 17). These support members 62, 63, 64 and 65 define a seating axis 73, as a bisector line between support members 62 and 64 on one side and support members 63 and 65 on the other side. The way in which the support members 62, 63, 64 and 65 define the seating axis 73 will be described in more detail below. The collimator 17, fibre optic cable 11, ferrule 21 and lens 23 as shown in Figure 10 are as described previously. Also shown in Figure 10 is a notional optical component 27 which can be considered to deflect the beam from lens 23 by some arbitrary angle, but which is notional in the sense that it is not necessarily an actual optical component but it is more intended to represent that the beam axis 71 of the output beam 72 from the collimator 17 need not be perfectly aligned with the longitudinal axis of the collimator 17, but could be at some arbitrary angle to that longitudinal axis. This angle does not matter since it is more important that the target for the output beam 72 is appropriately positioned to receive the output beam 72 and / or that the output beam 72 is appropriately steered by adjusting the beam steerer 30 so as to be directed correctly onto the intended target.

[0070] As depicted by the question mark in Figure 10, when the fibre barrel 10 (terminated by collimator 17) is being inserted into the detector head 4 with a push in the direction of arrow P, the precise orientation of the beam axis 71 relative to the components of the detector head 4 (for example represented by the seating axis 73) is somewhat uncertain.

[0071] With the previously-considered laser encoder system 1 as described above with reference to Figures 1 to 7, after inserting the collimator 17 into the detector head 4 and clamping it in place with the clamp 20 (in a somewhat uncertain orientation), a manual and time-consuming setup and alignment procedure would be performed in order to align the output beam 72 with the target optic 8.

[0072] As mentioned previously, the present applicant has appreciated that it can be challenging to achieve the correct alignment of the collimated beam from the collimator 17 when setting up the detector head 4, for example with the alignment being sensitive to the degree of tightening applied to the clamp 20 by the screw 22 and also being prone to drift should the clamp 20 become loosened over time.

[0073] The present applicant has also appreciated that it would be beneficial if the optical fibre assembly 32 could be easily removed and re-inserted into the detector head 4 without having to perform a complicated alignment setup each time.

[0074] Accordingly, the seating arrangement 60 used in an embodiment of the present invention is adapted and arranged to receive the collimator 17 in a stable and / or repeatable orientation. This is achieved by forming a kinematic coupling between the collimator 17 and the detector head 4 via the four support members 62, 63, 64 and 65 of the seating arrangement 60. In this embodiment, the outer surface of the collimator 17 is substantially cylindrical, so when it is seated onto the seating arrangement 60, the four support members 62, 63, 64 and 65 make contact with the collimator 17 respectively at four points of contact 82, 83, 84 and 85 as shown in Figure 11. These four points of contact provide kinematic or exact constraint in four corresponding degrees of freedom. What is meant by a kinematic coupling and kinematic constraint in this context will now be explained.

[0075] In the context of locating a first body relative to a second body, kinematic design considerations are met by constraining the degrees of freedom of motion of the first body relative to the second body using the minimum number of constraints (also referred to as exact constraint). Where there is over constraint in a coupling between the two bodies, i.e. where the coupling provides more than the minimum number of constraints by providing at least one redundant constraint, it is not possible to determine with any certainty which combination of constraints will determine the actual position of the first body relative to the second body. Accordingly, the position of the first body relative to the second body is not repeatable, because it is not known at which of the several possible positions the first body will come to rest relative to the second body when they come together again.

[0076] Furthermore, where there is over constraint, the first body does not assume a stable position relative to the second body because it may move between two or more of the different possible positions when a force is applied. An example of this is a four-legged table which will often rock between two different positions when placed on a flat surface. This is because the minimum number of constraints in this context is three, to constrain relative motion in three corresponding degrees of freedom (two rotational degrees of freedom and one translational degree of freedom), whereas a four-legged table has four constraints (created by contact between each of the four legs and the flat surface), such that the coupling between the table and the flat surface is over constrained.

[0077] In general, a single constraint is required for each degree of freedom to be constrained, and a single point contact between two bodies creates a single constraint. Therefore, to constrain a first body relative to a second body in all six degrees of freedom, the coupling between them would need to define six points of contact that are mutually arranged to constrain the two bodies relative to one another in six corresponding degrees of freedom without any redundancy. It will be understood that these points of contact need not be (and in practice would not be) mathematical points in the pure sense. Instead, in practice, each of these would typically be a small area that approximates a point. However, even though each point of contact may not be pure in a mathematical sense, the coupling can still be referred to as a kinematic coupling because it can still be considered to follow kinematic design principles. The term kinematic, as used herein in the context of a coupling between two bodies, is to be interpreted accordingly as meaning kinematic or at least pseudo-kinematic, and similarly for like terms such as kinematically.

[0078] In view of the above, it will be understood that use of a kinematic coupling between a first body and a second body provides a kinematically-defined position for the first body relative to the second body that is one of more of: unique, discrete, repeatable, consistent, predictable and stable (at least in respect of the degrees of freedom that are constrained by the coupling, i.e. ignoring any unconstrained degrees of freedom). Unless otherwise stated, a kinematic coupling as referred to herein is to be understood as providing relative constraint in all six degrees of freedom, with a kinematically-defined position being interpreted likewise as relating to a relative position defined kinematically in all six degrees of freedom. A kinematically-defined position can also be referred to in short as a kinematic location.

[0079] Because of its reliance on the principle of exact constraint, a kinematic coupling between two bodies is also by its nature readily couplable and decouplable. This is because, in order to prevent the two bodies from coming away from the constraints that define their relative position, it would be necessary to provide an additional (and opposing) constraint, such as a clamp, and the additional constraint would itself result in over constraint and would therefore turn it from a kinematic coupling into a non-kinematic coupling. That said, a kinematic coupling does typically require some form of nesting force (or biasing force) to hold the coupled bodies together, but without creating an additional constraint. In this respect, a theoretical constraint is not just a contact point alone, but also includes a corresponding nesting force that maintains the contact. The nesting force is a force vector that goes through the contact point normal to the surfaces of contact, but these can be vectorially combined into a single force. For example, a gravitational or magnetic force can be used as a nesting force.

[0080] By way of further background, these concepts are explored further in: (a) “Mechanical Design of Laboratory Apparatus” by H. J. J. Braddick, Chapman & Hall, London, 1960, pages 11-30; (b) “Exact constraint” by James G. Skakoon, Mechanical Engineering, September 2009; (c) “Kinematic Couplings: A Review of Design Principles and Applications” by Alexander Slocum, International Journal of Machine Tools and Manufacture 50.4 (2010): 310-327; and (d) “Principles and Techniques for Designing Precision Machines” by Layton C. Hale, Ph.D. thesis, Massachusetts Institute of Technology, February 1999.

[0081] In the context of the above explanation regarding kinematic design principles, the nesting force is provided by the arm 61 which is biased onto the surface of the collimator 17 by spring member 68 using a lever action (the spring member 68 and arm 61 form a biasing member). This does not provide a constraint as such, but merely ensures that the other constraints are in place. Leaving aside roll alignment features 66 and 67 for now, the seating arrangement 60 provides just four constraints, created respectively by the four contact points 82, 83, 84 and 85. These four constraints act to constrain relative movement between the collimator 17 and the detector head 4 (to which the seating arrangement 60 is fixedly coupled) in four corresponding degrees of freedom (two rotational and two translational degrees of freedom). Since there is exact constraint (or no over constraint), the coupling is kinematic (or at least pseudo kinematic) in nature, with the seating axis 73 being kinematically defined.

[0082] When considering a Cartesian XYZ coordinate system with the Z axis aligned with the seating axis 73, these four constrained degrees of freedom are: (a) up / down rotation relative to axis 73 (pitch, or rotation around X); (b) left / right rotation relative to axis 73 (yaw, or rotation around Y); (c) left / right translation relative to axis 73 (translation in X); and (d) up / down translation relative to axis 73 (translation in Y). There are two degrees of freedom that remain unconstrained: rotation around axis 73 (roll, or rotation around Z) and forward / backward translation along axis 73 (translation in Z).

[0083] Regarding the first unconstrained degree of freedom, rotation around axis 73 can be constrained if required by the post 67 shown on the lower surface of the collimator 17 in Figures 8 and 9, which is received into a slot 66 on the seating arrangement 60. This feature may also follow kinematic design principles by providing a single extra constraint (without over constraint) via a single point of contact. Alternatively, it could be provided merely to guide the collimator 17 loosely into a narrow range of roll angles if that is all that is required, in which case it should ideally not add any additional constraint because this would impact the kinematic nature of the constraint in the other degrees of freedom.

[0084] Regarding the second unconstrained degree of freedom, forward / backward translation along axis 73 is in practice constrained to some extent by the frictional contact between the arm 61 and the surface of the collimator 17. However, this constraint is not kinematic in nature, since it does not provide a stable and repeatable position for the collimator 17 along the seating axis 73, but it does at least offer some resistance to the collimator 17 from being moved axially except when this is intended e.g. when removing it from the detector head 4.

[0085] Figure 11 illustrates that, when the collimator 17 is seated into the seating arrangement 60 with all four points of contact 82, 83, 84 and 85 in operation, the beam axis 71 is now at a specific angle 9 relative to the seating axis 73. Although this angle 9 is not necessarily known, it is stable and repeatable. Therefore, should the fibre barrel 10 be removed from the detector head 4 and then reinserted, it will again come to rest on the seating arrangement 60 with the same angle 9 between the beam axis 71 and the seating axis 73. Because of this, although an initial alignment procedure may be required to align the output beam 72 relative to the target optic 8, having done so it would then be possible to remove the fibre barrel 10 from the detector head 4 and then later re-insert it, without having to perform the alignment procedure again.

[0086] Regarding roll alignment, the beam axis 71 can be arranged to be aligned with the seating axis 73, so that the roll angle does not affect the pointing direction of the output beam 72 (and so that the repeatability of the roll angle is not critical, only to a limited extend internally within the detector head 4 e.g. to ensure polarisation is at least approximately aligned with the relevant optical components). Alternatively, the roll constraint (i.e. the features 66 and 67) could be adapted to be kinematic in nature, to provide a repeatable roll alignment as well.

[0087] The use of a seating arrangement 69 such as described above is beneficial because not only does it achieve a stable and / or repeatable alignment of the collimator 17 within the detector head 4, it does so by a simple push-in action without any complicated mechanism that might be prone to human error. The tightening applied to the clamp 20 by the screw 22 in the previously-considered design as shown in Figure 7 can be problematic because the relatively high force exerted on the fibre barrel 10 can lead to unpredictable behaviour particularly in respect of shock and thermal changes during subsequent use, which can in turn lead to undesirable changes in alignment (and also therefore changes in signal strength, with the signal in this context being the signal output from the detector head 4).

[0088] It should be noted that, even with a light tightening of the screw 22 (or relying only on the biasing force resulting from spring 39), the design of Figure 7 would still not have the same benefits as an embodiment of the present invention because the contact between the collimator 17 and the seating surface is over constrained (it does not provide kinematic or exact constraint). Therefore, there is uncertainty regarding how the collimator 17 is seated and there may be movement of the collimator 17 during subsequent between different ones of the multiple available positions due to the over constraint.

[0089] With an embodiment of the present invention, the optical fibre assembly 32 can be easily removed and re-inserted into the detector head 4 without having to perform a complicated alignment setup each time. An embodiment of the present invention also makes it possible to insert the same optical fibre assembly 32 into different detector heads 4 of the same type or different optical fibre assemblies 32 of the same type into the same detector head 4, all without having to perform a complicated alignment each time, so that the system is more modular, flexible and interchangeable. It will be appreciated that in practice, due to slight variations in the stop position (both axially and in terms of roll) and slight variations in manufacturing (for example variations in the surface of the collimator 17 and / or variations in the pointing direction of the beam output from the collimator 17) there might still be a slight variation in alignment, so that the alignment is not perfectly repeatable and predictable in a pure theoretical sense, and so that minor adjustments in alignment might still be required to optimise the signal strength after installing the optical fibre assembly 32 into the detector head 4. However, it is far easier for the operator to start with an alignment is close to where it should be (with a strong signal), rather than start from a point where the signal strength is weak and so that large adjustments are required to recover a strong signal. If more precise repeatability and interchangeability is important in a particular use case then this can be achieved with an embodiment of the present invention by a more precise manufacture of the relevant parts (e.g. to ensure that the surface of the collimator 17 is more perfectly cylindrical), and for example by providing kinematic constraint in more than just four degrees of freedom (for example in all six degrees of freedom). In any case, the kinematic nature of the coupling also provides stability for the seating position of the optical fibre assembly 32 in the detector head 4, which is an important benefit of an embodiment of the present invention independently of the benefit of repeatability.

[0090] Although the above embodiment makes use of four support members 62, 63, 64 and 65 to provide four corresponding constraints (and thereby kinematic constraint in four degrees of freedom), if a simpler implementation is required then this could be relaxed such that the seating axis 73 is defined by just three support members 62, 63 and 64 as shown in Figure 12. However, the use of four contacts is preferred as it provides more stability and repeatability.

[0091] It will also be appreciated that the collimator 17 could be supported on pads or rollers rather than on spheres. In each of these cases, contact is made over a relatively small area (approximated as a point) between the cylindrical surface of the collimator 17 and the support member concerned. For example, a cylindrical support member would be arranged at right angles to the collimator 17 to create a point-like contact between the two surfaces, with the support members being angled inwards in pairs to form two v-groove features. In the case where the support members are pads, there would either be a line contact between the surface of the collimator 17 and each pad, or each pad could be shaped to present a slightly concave surface to the collimator 17 thereby to create a more point-like contact. Although any of these types of support member would be suitable, the use of pads might be preferred in some circumstances due the ease of manufacturing and thermal stability. Also, instead of a plain cylinder bearing onto four pads, the collimator 17 could instead have two raised annular rings which seat into an elongate v-groove to create the four points of contact.

[0092] Figures 13 to 17 show several views of an embodiment of the present invention that is based very closely on that shown somewhat more schematically in Figures 8 to 12, so that a detailed further description is not required (like reference numerals refer to like parts). Figure 13 shows a perspective view of the collimator 17 biased onto the seating arrangement 60 by arm 61 and spring 68. Also shown in Figure 13 are a detent feature 49 and cams A and B; these features are not described further herein but are explored in our co-pending application filed on the same date as the present application. Figure 14 shows a view from a slightly different angle to that of Figure 13 and also shows the strain relief 15 and part of the housing 14.

[0093] Figure 15 show a view looking down on the seating arrangement 60 with the collimator 17 removed. It can be seen that, in this embodiment, each of the support members 62, 63, 64 and 65 is in the form of a pad rather than a sphere, with the surface of each pad being raised above a surrounding surface 69 so that the collimator 17 makes contact with the detector head only via the support members 62, 63, 64 and 65 (to create exact or kinematic constraint), with the biasing member 61, 68 not providing a nesting force as described above. Figure 15 also shows the slot 66 into which the post 67 on the underside of the collimator 17 will be guided as the collimator 17 is pushed into the detector head 4, thereby providing roll alignment for the collimator 17.

[0094] Figure 16 is an end view similar to Figure 9, but looking the other way (in a direction from inside the detector head 4 to out), showing how the collimator 17 sits on the pads 64, 65 and also showing how the post 67 on the lower surface of the collimator 17 is received into the slot 66 on the seating arrangement 60 to provide constraint (or at least some limits) on rotation (or roll) around the longitudinal axis of the collimator 17. In the view shown in Figure 16 there is clearance between the post 67 and the walls of the slot 66, such that no additional contact has been created which might impact the kinematic (or pseudo kinematic) constraint provided in the other degrees of freedom (i.e. that provided via pointlike contact between the collimator 17 and the pads 64, 65). Therefore, I nthis example the post 67 and slot 66 are provided merely to guide the collimator 17 into a narrow range of roll angles (purely by way of example, the lateral movement of post 67 within the slot 6 could even be as small as 80 pm). It would also be acceptable in practice if there were some residual (though unbiased) contact between the post 67 and the walls of the slot 66 after the post 67 has been guided into place by the angled side walls of slot 66 (see Figure 15).

[0095] Alternatively, a sideways biasing (or nesting) force could be provided on the post 67 to bias it into contact with the opposite side wall of slot 66, which would create a kinematic (or pseudo kinematic) constraint in this rotational degree of freedom, by virtue of the small contact area between the post 67 and slot 66.

[0096] Figure 17 is a view to help explain calculation of the spring force of spring member 68 and the lever distance for arm 61 to ensure that the optical fibre assembly 32 is held against gravity by the frictional resistance provided by arm 61 even when the detector head 4 is oriented vertically. Purely by way of example, if the mass of the collimator 17 (and other supported parts of the optical fibre assembly 32) is assumed to be 24 g and the coefficient of friction (between the collimator 17 and the arm 61) is estimated to be 0.2, then the force required to stop the collimator 17 sliding would be (0.024 x 9.81 / 0.2) = 1.18 N. On this basis, it would be appropriate to choose a spring member 68 which provides a force of 14.2 N (at the relevant extension), along with a lever ratio (LI / (LI + L2)) of (11 / 19) because this would provide a holding force of (14.2 x 11 / 19) = 8.2 N, thereby giving a factor of safety of (8.2 / 1.18) = 7, i.e. the holding force is 7 times what is required and thereby provides a margin of safety. In the embodiments described above, the collimator 17 is described as being kinematically (or at least pseudo kinematically) constrained relative to the detector head 4 in four degrees of freedom (two rotational degrees of freedom and two translational degrees of freedom), with the option of adding kinematic constraint in the third and final rotational degree of freedom (thereby creating overall kinematic constraint in five degrees of freedom). By providing a suitable stop member for the optical fibre assembly 32 when it is pushed into place, it would be possible to extend the kinematic constraint to all six degrees of freedom. For example, a substantially planar end wall could be provided at the far end of the slot 66 shown in Figure 15, such that when the post 67 shown in Figure 16 is pushed up against the end wall a point-like contact would be created. A suitable biasing (or nesting) force would ensure that contact is maintained to ensure the integrity of the kinematic constraint.

Claims

CLAIMS1. A laser encoder device into which an optical fibre assembly is installable, the device comprising a seating arrangement for receiving and holding a component of the assembly within the device, and the seating arrangement being adapted to constrain the component kinematically relative to the device such that the assembly can be removed from and reinstalled into the device with a stable and / or repeatable alignment of the component relative to the device.

2. A device as claimed in claim 1, wherein the seating arrangement is adapted to constrain the component kinematically in at least three degrees of freedom.

3. A device as claimed in claim 1 or 2, wherein the seating arrangement is adapted to constrain the component kinematically in at least four degrees of freedom.

4. A device as claimed in claim 1, 2 or 3, wherein the seating arrangement is adapted to constrain the component kinematically in at least two rotational and at least two translational degrees of freedom.

5. A device as claimed in claim 4, wherein the seating arrangement is adapted to constrain the component kinematically in all three rotational degrees of freedom.

6. A device as claimed in claim 4, wherein the seating arrangement is adapted to hold the component within a predetermined range of angles in a third rotational degree of freedom without impacting the kinematic constraint in the other degrees of freedom.

7. A device as claimed in claim 5 or 6, wherein constraint in the third rotational degree of freedom is provided by a post projecting from a surface of the component which is received into a slot in the seating arrangement.

8. A device as claimed in any preceding claim, wherein the kinematic coupling is unconstrained in a translational degree of freedom to allow the assembly to be installed into and removed from the device.

9. A device as claimed in claim 8, wherein at least some resistance to translation in the unconstrained degree of freedom is provided by a frictional force from a biasing member which provides a nesting force for the kinematic coupling.

10. A device as claimed in claim 9, wherein the frictional force is arranged to be greater than a gravitational force due to the weight of the component and anything supported by it.

11. A device as claimed in claim 9 or 10, wherein the biasing member is arranged to move out of the way of the component as it is installed into the device.

12. A device as claimed in any one of claims 8 to 11, wherein the unconstrained translational degree of freedom defines a seating axis for the seating arrangement, and wherein the alignment of the component relative to the seating axis is repeatable with each reinstallation of the assembly into the device.

13. A device as claimed in any one of claims 8 to 12, when dependent on claim 6, wherein a translational axis of the unconstrained translational degree of freedom is aligned with a rotational axis of the third rotational degree of freedom.

14. A device as claimed in any preceding claim, wherein the device comprises an opening through which the assembly is inserted to install it into the device.

15. A device as claimed in claim 13, when dependent on claim 8, wherein the assembly is inserted into the device in a direction that is substantially aligned with the unconstrained translational degree of freedom.

16. A device as claimed in any preceding claim, wherein the laser encoder device is a detector head.

17. A device as claimed in any preceding claim, wherein the assembly comprises a fibre optic cable for providing light to the device in use.

18. A device as claimed in any preceding claim, wherein the component terminates the assembly.

19. A device as claimed in any preceding claim, wherein the component is an optical or metrology component.

20. A device as claimed in any preceding claim, wherein the component is a collimator.

21. A device as claimed in claim 20, wherein the collimator is coupled to a strain relief component.

22. A device as claimed in claim 21, wherein the collimator and strain relief component form a fibre barrel component.

23. A method of installing an optical fibre assembly into a laser encoder device as claimed in any preceding claim, comprising inserting the assembly into an opening in the device such that the component is coupled kinematically into the device via the seating arrangement, removing the assembly from the device, and inserting the assembly into the device again such that the component is again coupled kinematically into the device via the seating arrangement with a repeatable alignment of the component relative to the device.

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